The present invention relates to an oxygen storing material and a process for preparation thereof. In another aspect, the present invention relates to the use thereof for the catalytic conversion of materials, in particular for the exhaust gas purification treatment of internal combustion engines.
Internal combustion engines emit in the exhaust gas, as harmful substances, essentially carbon monoxide CO, unburnt hydrocarbons HC and nitrogen oxides NOx, a high percentage of which are converted by modern exhaust gas treatment catalysts into harmless components; namely, water, carbon dioxide and nitrogen. The reaction takes place under substantially stoichiometric conditions, that is the oxygen contained in the exhaust gas is controlled using a so-called lamda probe so that the oxidation of carbon monoxide and hydrocarbons and the reduction of nitrogen oxides to nitrogen can take place approximately quantitatively. The catalysts developed for this purpose are known as three-way catalytic converters. They usually contain, as catalytically active components, one or more metals from the platinum group in the Periodic Table of Elements deposited on high surface area support materials such as xcex3-aluminum oxide with specific surface areas of more than 100 m2/g.
Stoichiometric conditions prevail with normalized air/fuel ratios xcex of one. The normalized air/fuel ratio xcex is the air to fuel ratio normalized to stoichiometric conditions. The air/fuel ratio states how many kilograms of air are required for complete combustion of one kilogram of fuel. In the case of conventional gasoline fuels, the stoichiometric air/fuel ratio has a value of 14.6. The engine exhaust gas, depending on the load and speed, exhibits larger or smaller periodic variations in normalized air/fuel ratio around the value 1. For better conversion of the oxidizable, harmful components under these conditions, oxygen storing components such as, for example, cerium oxide are used which bond the oxygen by changing the oxidation state of-the cerium from Ce3+ to Ce4+ when it is present in excess and release it again for oxidative conversion by changing from Ce4+ back to Ce3+ when there is deficiency of oxygen in the exhaust gas.
Automotive exhaust gas catalysts operate with exhaust gas temperatures of up to 1100xc2x0 C. These high temperatures require the use of catalyst materials, which have appropriate thermal resistance and long term stability.
EP 0 444 470 B1 describes a high surface area cerium oxide which consists of an intimate mixture of cerium oxide with 5 to 25 mol. %, with respect to the moles of cerium oxide, of a cerium oxide stabilizer. Lanthanum, neodymium and yttrium are mentioned as stabilizers. The material is obtained by coprecipitation from a common solution of a cerium oxide precursor and a precursor for the cerium oxide stabilizer.
According to EP 0 715 879 A1, the oxygen storing capacity of cerium oxide can be used to the optimum extent if it is deposited in the form of particles with diameters of 5 to 100 nm on a porous support material such as, for example, aluminum oxide. For this purpose, a dispersion of the powdered support material and a cerium oxide sol which has particle diameters in the cited range is made up. A honeycomb carrier structure is coated with this dispersion and then dried and calcined for a period of 1 hour at 650xc2x0 C. A cerium oxide sol together with a zirconium oxide sol may also be used. As a result of calcination, the particle sizes of the cerium oxide on the support material are increased to more than 35 nm. If cerium oxide sol and zirconium oxide sol are used together, then a solid solution of cerium oxide and zirconium oxide with particle sizes of about 60 nm is formed as a result of calcining the coating (750xc2x0 C., 1 h).
EP 0 337 809 B1 describes a catalyst composition which contains inter alia zirconium oxide particles stabilized with cerium oxide. The zirconium oxide particles are stabilized with cerium oxide by soaking zirconium oxide with a cerium salt solution. The soaked particles obtained in this way are dried and calcined until a graphical representation of the X-ray diffraction pattern no longer shows a peak for the crystalline form of cerium oxide. The cerium oxide is present in the cerium oxide/zirconium oxide mixture in an amount of 10 to 50 wt. %, with respect to zirconium oxide. In addition to the cerium salt, an yttrium and/or calcium salt may also be used. After calcination for 10 hours in air at a temperature of 900xc2x0 C., the X-ray diffraction pattern of the material shows only a peak for tetragonal zirconium oxide and no peak for cerium oxide. Cerium oxide is thus present in this material substantially in the form of a solid solution with the zirconium oxide.
The processes known from the prior art for preparing an oxygen storing material thus use coprecipitation processes and impregnation processes in order to stabilize cerium oxide by adding other components or to deposit cerium oxide on support materials. Alternatively, cerium oxide may be deposited directly onto the support materials in particle form by using a cerium oxide sol.
The disadvantage of a coprecipitation process is the fact that the material obtained has too high a percentage of cerium oxide which cannot be fully utilized for the task of oxygen storing because the oxygen storing process takes place substantially at the surface and thus the deeper lying regions within the material are not available for storage.
In the case of known impregnation processes or the deposition of sols, chromatographic effects occur during dewatering of the treated material and these lead to non-uniform distribution of the cerium oxide on the support material. It has been shown that pore volume impregnation, which avoids chromatographic effects, also leads to unsatisfactory results because only a volume of solvent which corresponds to the water absorption capacity of the support material is used. In addition the volume of solvent in this process is restricted by the water absorption capacity so that it is not possible to dissolve completely variable amounts of cerium salts therein.
Van Dillen et al (Proc, 6th Conf. on Cat., London, ed., G. C. Bond, P. B. Wells, F. C. Tomkins, 2667 (1976)) describe a process for preparing copper and nickel catalysts on high surface area support materials. In this process the support material, together with a precursor for the active component, is dispersed in water. The active component is precipitated onto the surface of the support material by injection of a basic or acidic solution into the dispersion using a capillary (capillary injection). In order to avoid rapid precipitation of the active component in the solution itself, precipitation has to be performed with only slight supersaturation of the entire dispersion. In order to ensure homogeneous precipitation in the entire solution, the basic or acidic solution has to be introduced in small amounts per unit of time and distributed uniformly by stirring the dispersion.
An object of the present invention is to provide an oxygen storing material which is characterized by high thermal stability and long term stability and which can be prepared in a simple manner.
In achieving this and other objects, one feature of the present invention is an oxygen storing material with high thermal stability which contains cerium oxide and at least one stabilizer selected from the group consisting of praseodymium oxide, lanthanum oxide, yttrium oxide and neodymium oxide, wherein the stabilizer(s) are present in highly dispersed form on the surface of a high surface area support material and the oxygen storing material still has a specific surface area of more than 20, preferably more than 30 m2/g, after calcination in air at 900xc2x0 C. for a period of 10 hours.